You can write and select anything what is changeable in the page.
When you are done with it and want to change the page on the server, you must fill in the password near the bottom of the page and Submit request.
If you don't fill the password or fill an invalid password, you get Sample of page changes, showing how the changed page would look like, but nothing is actually changed on the server. All other visitors of the site continue to see the original contents of the page. You can use the Submit request without password to first check your changes (e.g. the selectable background), before you make them permanent on the server and visible to other visitors of the site.
Until you supply a valid password for the page, you cannot change anything and you can play with the page without any possibility of any disturbance.